TRANSACTIONS OF SECTION K. 1029 



applied equally to such parts as are connected by lineal descent, and also to those 

 which we . have good reason to believe have resulted from parallel development 

 in quite distinct phyla ? Or, to put a finer point upon our inquiry, are we to 

 distinguish in any way the cases of ' individual repetition ' from those of ' essential 

 correspondence ? ' In the latter case I think no good end would be served at pre- 

 sent by accentuating this distinction by tei-ms : tlie steps of divergence are so 

 slight and gradual. None the less should it be clearly borne in mind that compari- 

 sons of parts commonlj^ ranked as homologous in the plant body are based on a less 

 complete individual correspondence than that of parts usually compared in the 

 animal body. 



But the case is different in dealing with parallel developments, and some doubt 

 arises whether parts which probably, or it may be certainly, have arisen by 

 separate evolutionary sequence in distinct phyla are to be classed as homologous 

 in the same sense as those directly related by descent. This question was long ago 

 taken up on the zoological side by Professor TJay Lankester, and it was shown 

 that the old word ' homology ' covered two things recognised as distinct from the 

 point of view of descent. He defined as homogenous ' structures which are 

 genetically related, in so far as they have a single representative in a common 

 ancestor.' On the other hand, ' when identical or nearly similar forces or environ- 

 ments act on two or more parts of an organism which are exactly or nearly alike : 

 further, if, instead of similar parts in the same organism, we suppose the same 

 forces to act on parts in two organisms, which parts are exactly or nearly alike, 

 and sometimes homogenetic, the resulting correspondences called forth in the 

 several parts in the two organisms will be nearly or exactly alike. ... I propose 

 to call this kind of agreement ho7n6plasis or homdplasy.' Now this distinction of 

 terms requires also to be observed in plant-morphology, and I am surprised that 

 it has never yet been adopted by botanists, though we have long recognised cases 

 of parallel development. I do not propose now to spend time in assigning these 

 terms to familiar cases : but to take tbe examples .already cited, the leaf of a Fern 

 would be homoplastic, though not homogenetic with the leaf of a Moss ; or, taking 

 examples from plants more nearly akin, it would appear possible that the leaves of 

 the three distinct phyla of living Pteridophytes show merely homoplasy, not a true 

 homogeny. 



The successive foliage leaves of most plants are assumed in the individual to 

 be the result of a mere repetition of development. But it is quite a possible view 

 that in the plant-body (as is contemplated in the animal in those cases of 'serial 

 homology ' which Lankester recognises as homoplastic) homoplasy may have had 

 a place. We must inquire whether all those structures which we designate 

 ' leaves ' have actually been the result of a development identical, or at least 

 essentially similar as regards their origin in the race. The problem is, given a 

 plant with numerous leaves of various form and function, to unravel the real story 

 of their evolution. Two distinct factors may be contemplated as possibly occurring 

 even in the individual, viz. : 



1. Homogeny of genetically related parts, with or without repetition of the 

 parts formed. 



2. Homoplastic an origin of two or more distinct categories of parts, not 

 genetically related, on the same organism. 



Working upon either of these, and thus complicating the problem by oblitera- 

 ting such distinctions as may have existed at first, may be the phenomenon of 

 metamorphosis. This has lately received its evolutionary definition at the hands of 

 Professor Goebel, as re.stricted to those cases where there has been an obvious 

 change of function. We see how change of function accounts for various forms 

 of leaf in certain cases; but it does not follow that all leaf-forms on the same 

 plant were so produced, by metamorphosis of a single original type. 



The Lycopodineffi are particularly interesting in illustration of this point. It 

 appears probable that Phylloglossian is a more primitive type than other living 

 Lycopods ; it has two kinds of leaf, the protophylls borne in irregular number and 

 arrangement on the protocorm, and the sporophylls of different form from these, 

 and arranged regularly on the strobilus : commonly there are no intermediate 



